1. Field of the Invention
This invention relates to a space photovoltaic generation system wherein sunlight is converted into electric energy in space and the electric power is transmitted by microwave, etc., and is received at a power base for use as electric energy.
2. Description of the Related Art
Because of finitude of electric energy based on the fossil fuels of oil, coal, natural gas, etc., and adversely affecting the environment, attention is focused on sunlight as an energy source to replace the electric energy based on the fossil fuels. Ground photovoltaic generation, etc., exists as one mode of electric energy use based on sunlight, but it is hard to stably supply electric power because of the sunshine amount between day and night, the effect of weather, etc., and the efficiency is poor. On the other hand, atmospheric attenuation scarcely exists in space and the solar energy density in space even in the vicinity of the earth reaches five to 10 times that on the ground; the lure of solar energy use in space is large. Research and development on a space photovoltaic generation system wherein solar energy in space is converted into electric energy and the electric energy is transmitted by microwave, etc., and is received at a specific location is underway.
As an example of a related art of such a space photovoltaic generation system, FIG. 9 is drawing to show the configuration of a space photovoltaic generation system in a related art in xe2x80x9cU.S.DOE and NASA Reference System Report, xe2x80x9cSatellite Power System: Concept Development and Evaluation Programxe2x80x9d, DOE/ER-0023, 1978.xe2x80x9d In FIG. 9, numeral 4 denotes a power generation satellite, numeral 5 denotes a photoelectric conversion unit formed of solar cell panels installed in the power generation satellite 4, numeral 9 denotes a transmitting antenna mounted on the power generation satellite 4, numeral 10 denotes a microwave radiated from the transmitting antenna 9, numeral 11 denotes a power base, and numeral 12 denotes a receiving antenna placed in the power base 11.
In the space photovoltaic generation system shown in FIG. 9, the photoelectric conversion unit 5 installed in the power generation satellite 4 performs photoelectric conversion of sunlight. The generated power energy is transmitted through the transmitting antenna 9 to the power base 11 as the microwave 10 and is received at the receiving antenna 12 in the power base 11. In the example cited as the related art, the photoelectric conversion unit 5 installed in the power generation satellite 4 has a size of 5xc3x9710 km, the transmitting antenna 9 has a diameter of 1 km, and the receiving antenna 12 in the power base 11 has a size of 10xc3x9713 km. The power generation satellite 4 has a weight of 50000 tons. The total size of the solar cell panels forming the photoelectric conversion unit 5 is determined in response to the amount of electric power generated by the power generation satellite 4, and the sizes of the transmitting antenna 9 and the receiving antenna 12 are determined in response to the receiving power efficiency.
Here, defining the value of normalizing electric power Prx arriving at the aperture area of the receiving antenna 12 having an aperture diameter Drx based on electric power Ptx transmitted through the transmitting antenna 9 having an aperture diameter Dtx as receiving power efficiency xcex7b, if distance d between the transmitting antenna 9 and the receiving antenna 12 is sufficiently long so as to form a Fraunhofer region (region assumed to be electrically infinite distance) and the aperture distribution of the transmitting antenna 9 is uniform in both amplitude and phase, radiation field distribution E of the transmitting antenna 9 and the receiving power efficiency xcex7b are represented by the following expressions:                     E        =                                            J              1                        ⁡                          (                              Z                θ                            )                                            Z            θ                                              (        1        )                                          η          b                =                                            P              rx                                      P              tx                                =                                                                      ∫                  0                  θ                                ⁢                                  |                  E                  ⁢                                      |                    2                                    ⁢                                                            Z                      θ                                        ⁢                                          xe2x80x83                                        ⁢                                          ⅆ                                              Z                        θ                                                                                                                                          ∫                  0                  π                                ⁢                                  |                  E                  ⁢                                      |                    2                                    ⁢                                                            Z                      θ                                        ⁢                                          xe2x80x83                                        ⁢                                          ⅆ                                              Z                        θ                                                                                                                  =                          1              -                                                J                  0                  2                                ⁡                                  (                                      Z                    θ                                    )                                            -                                                J                  1                  2                                ⁡                                  (                                      Z                    θ                                    )                                                                                        (        2        )                                          Z          θ                =                  π          ⁢                                    D              tx                        λ                    ⁢          sin          ⁢                      xe2x80x83                    ⁢                      (            θ            )                                              (        3        )                                θ        =                              tan                          -              1                                ⁡                      (                                          D                rx                                            2                ⁢                d                                      )                                              (        4        )            
where xcex is the wavelength of the microwave 10 and Jn (x) is a Bessel function of the order n. From expression (2), it is seen that the aperture diameters of both the transmitting antenna 9 and the receiving antenna 12 need to be made large to enhance the receiving power efficiency xcex7b. If the transmitting antenna 9 and the receiving antenna 12 differ in aperture shape or aperture distribution, the calculation expression of the receiving power efficiency xcex7b also varies accordingly. However, if the aperture diameter of the transmitting antenna 9 or the receiving antenna 12 is made large, the receiving power efficiency xcex7b is always enhanced.
If the distance d between the transmitting antenna 9 and the receiving antenna 12 is sufficiently large as compared with the aperture diameter Drx of the receiving antenna 12, the following expression holds according to expressions (3) and (4):                               Z          θ                ≅                  π          ⁢                                                    D                tx                            ⁢                              D                rx                                                    2              ⁢                              xe2x80x83                            ⁢                              λ                ⁢                d                                                                        (        5        )            
From expression (5), if either of the aperture diameters of the transmitting antenna 9 and the receiving antenna 12 required for achieve one receiving power efficiency is determined, the aperture diameter of the other is also determined. To provide high receiving power efficiency, the aperture diameter of the transmitting antenna 9 or the receiving antenna 12 needs to be made large. FIGS. 10(a) and 10(b) show the characteristics of the radiation field distribution of the transmitting antenna 9 in the Fraunhofer region and receiving power efficiency if the wavelength xcex of the microwave 10 radiated from the transmitting antenna 9 is 52 mm (frequency 5.8 GHz). From the figures, it is seen that, for example, if the power generation satellite 4 is placed in stationary orbit above the ground of 36000 km and the aperture diameter of the transmitting antenna 9 is 1 km and the aperture distribution is uniform, the aperture diameter of the receiving antenna 12 needs to be about 7 km to provide receiving power efficiency 90%.
From expression (2), if the transmission frequency of the microwave 10 radiated from the transmitting antenna 9 is made high (the wavelength is shortened), the aperture diameter of the transmitting antenna 9 or the receiving antenna 12 can be lessened, but a problem of interfering with the frequency bands used with the already existing satellite communications, ground microwave communications, etc., is involved. To place the power base 4 on the ground, generally as the frequency becomes high, an atmospheric loss cannot be ignored and the receiving power efficiency is lowered. Thus, the frequency range used for the microwave 10 is limited. 2-GHz band (2.45 GHz) and 5-GHz band (5.8 GHz) are named as the frequencies assumed in the space photovoltaic generation system so far.
To increase the amount of electric power generated by the power generation satellite 4, the area of the solar cell panels forming the photoelectric conversion unit 5, a reflecting mirror for condensing sunlight, or the like needs to be increased.
By the way, the power generation satellite installing the solar cell panels and the transmitting antenna needs to be hoisted into predetermined orbit in space using a rocket or a shuttle. On the other hand, the dimensions and weight that can be carried in a rocket, etc., are limited and thus if the dimensions or weight of the solar cell panels and the transmitting antenna contained in the power generation satellite are large, it is physically difficult to hoist and develop them into space at a time.
Then, a method of launching the components of the power generation satellite more than once is possible. In this case, however, it is necessary to assemble the components in space or at a similar altitude and then hoist the power generation satellite into predetermined orbit. Also in this case, if the dimensions or weight of the solar cell panels and the transmitting antenna contained in the final power generation satellite are large, it becomes necessary to launch the components a large number of times, bearing the costs is large, and the time period to the actual operation of the power generation satellite is also long; the barrier against realizing the method is high. Further, there is a problem of complicating electric, mechanical, and thermal interfaces to assemble the components and to assemble the components into the power generation satellite at a lower altitude than the Van Allen belt, when the power generation satellite passes through the Van Allen belt, the electronic machines, the solar cell panels, etc., are broken and are degraded in performance due to the effect of radiation, etc.; this is also a problem.
The transmitting antenna mounted on the power generation satellite needs to transmit a microwave precisely to the target power base. If the attitude of the power generation satellite is controlled with very high accuracy, there is no problem. However, if the distance between the power generation satellite and the power base is very long or if the aperture area of the receiving antenna in the power base is small, the beam direction from the transmitting antenna needs to be controlled independently of the attitude of the power generation satellite. To satisfy such requirement, a method of adopting an array antenna as the transmitting antenna and electrically scanning a beam is possible. However, not to produce an unnecessary beam called grating lobe, generally element antennas need to be arranged with spacing of one wavelength or less throughout the antenna aperture. If the aperture area of the transmitting antenna is very large, a large number of element antennas need to be arranged in proportion to the very large aperture area. For example, assuming that the wavelength of the microwave radiated from the transmitting antenna is 52 mm (frequency 5.8 GHz) and that the aperture diameter of the transmitting antenna is 1 km, if the element antennas are arranged with spacing of one wavelength, the number of the element antennas reaches about 290 millions. Therefore, the scale of manufacturing and assembling the transmitting antenna is very large and the manufacturing difficulty also becomes high; this is a problem.
On the other hand, if the aperture area of the receiving antenna is made large, the aperture area of the transmitting antenna can be lessened. However, to place the power base on the ground, from the viewpoint of securing land, it becomes difficult to secure an enormous area physically and in cost. To place the power base in space or on the moon, a similar problem to that involved in launching the power generation satellite mentioned above still arises.
It is therefore an object of the invention to provide a space photovoltaic generation system for making it possible to minimize the scale of a receiving antenna in a power base without impairing the receiving power efficiency unnecessarily if the transmitting antenna mounted on each power generation satellite is made small to remove the problems involved in the large scale of the transmitting antenna mounted on each power generation satellite in the related art.
According to the invention, there is provided a space photovoltaic generation system including a plurality of power generation satellites and a power base. Each of power generation satellites has a photoelectric conversion unit, a transmission frequency conversion unit, a microwave control unit, and a transmitting antenna. The photoelectric conversion unit converts sunlight into electric energy. The transmission frequency conversion unit performs frequency conversion of the electric energy provided by the photoelectric conversion unit to a microwave. The microwave control unit controls at least one of the amplitude and the phase of the microwave output by the transmission frequency conversion unit. The transmitting antenna radiates the microwave. The power base has a receiving antenna and a reception frequency conversion unit. The receiving antenna receives the microwave radiated from the power generation satellites. The reception frequency conversion unit performs frequency conversion of the microwave received at the receiving antenna to one of DC and low-frequency commercial power. The plurality of power generation satellites are placed in space to form a power generation satellite group. An array antenna having the transmitting antennas of the power generation satellites in the power generation satellite group as element antennas is formed.